Well we completely unexpectedly wandered into a chance to ride both the F800 GS and a GS1200 two up one after the other and I gotta say that my wife LOVED the pillon on the 800, the 1200 has a slope on the pasenger seat that kept her sliding into me, the 800 was MUCH more comfortable. We also found the ride to be WAYYYYY more compliant on the 800 and the power was more than acceptable. The 1200 is faster, but imho I could ride the 800 more quickly and into more places than I would enjoy on the big gs..

I think for 2 up traveling, the 1200 would be better, but if you're just planning on riding around and not carrying camping gear, the 800 would suit you fine.
Gas mileage is not an issue.

Long haul two up, 1200 for sure. Camping gear on the other hand is no problem, have done one trip fully loaded and also some two up. It's got enough guts for carrying gear, but definitely not a two up touring bike.

Having just sold my Aprilia Capo & then purchased an 800GS I'd agree with the earlier post. Check out the Triumph Tiger. I do 2-up on the GS with no issues but with slightly more power (although the bike is heavier too) and more comfy seat along with more street orientation of the Tiger, it would be a better 2-up asphalt bike. Consider the Caponord too depending on which brand has dealer support closer to you. Either can be found for great deals in the used market. I'm gonna keep my 800GS for a while but may sell my Triumph Rocket III and try a Tiger for the added comfy issue with my wife.
If you are really concerned with MPG, we get about 42 on the 800GS and just a couple ticks lower on the Capo when we 2-up. Both are about 6 higher when alone.